I don't think a retirement stipulation is necessary for the Undertaker. However if that was the stip, would you have just a regular singles match? Or have him be buried alive or shut into a casket thus forcing his retirement?

I'd say have him lose a normal match at Survivor Series, the site of his on-camera debut. After losing, as his opponent makes his way up the ramp, the purple lights turn on, and fog starts coming out around the ring.

Once the opponent leaves for good, the camera focuses on Taker lying in the ring as the fog envelops him. The lights go out. After a few seconds, the lights come on fully. His body is gone, but an urn sits in the ring. End broadcast.

Buried alive and casket matches would make u think/expect he will lose couse its a type of match some1 like UT could rly go out on but i think a face will end his streak and UT will give him his respect and make him a big thing even tho it will probably backfire and the dude will get MEGA HEAT till he retires xD